(i) Field of the Invention
This invention relates to coating compositions and more particularly to decorative coating compositions that can be rapidly cured to urethane coatings at ambient temperature.
(ii) Description of the Prior Art
It is well established that polyisocyantes react with active hydrogen compounds, for example, polyols to form urethanes. Urethane based coatings are well known for their toughness, chemical resistance and durability.
The reaction is illustrated by Equation I: ##STR1## wherein R is the backbone of the polyol and R.sup.1 is the backbone of the polyisocyanate.
The reaction is accelerated in the presence of a tertiary amine. The reaction in the presence of a tertiary amine is believed to proceed via the formation of an intermediate with the conversion of the intermediate to the urethane involving the formation of an orthoester type structure followed by the expulsion of the amine.
One possible reaction mechanism is illustrated in Scheme (I) below, employing triethylamine as the tertiary amine: ##STR2##
A second possibility is shown in Scheme II: ##STR3##
These mechanisms suggest that the more active the hydrogen of the hydroxyl group in the polyol, i.e., the more acidic is the hydrogen, the faster will be the reaction with the di- or polyisocyanates to form the cured urethane, in the presence of the tertiary amine vapour. The presence of free carboxyl groups in the coating composition is not good since the resultant product formed with the isocyanate is unstable and carbondioxide is evolved in the form of bubbles which produce a non-smooth, bubbled surface, in accordance with Equation II: ##STR4## Among the organic hydroxyl group-containing compounds, aromatic hydroxyl compounds, for example, phenolics have more acidic hydroxyl hydrogen atoms than do aliphatic hydroxyl compounds, because phenoxide ions are resonance stabilized for example, a phenoxide ion is resonance stabilized in accordance with structures 1, 2 and 3: ##STR5##
Hence, phenolic resins, for example, novolacs and resoles, having phenolic hydroxyl groups available for cross linking with free isocyanate groups would appear to be suitable for reaction with di- and poly-isocyanates to give urethanes.
This type of reaction has been taken advantage of in the preparation of foundry resins in which the resin is cured at room temperature by employing a tertiary amine as catalyst.
This is illustrated in Canadian Pat. No. 903,944, issued June 27, 1972 and Canadian Pat. No. 816,762, issued July 1, 1969, both of Janis Robins.
Although resin compositions based on phenolic resin components and di- or poly-isocyanate can be rapidly cured at room temperature in the presence of a tertiary amine as catalyst, they have not been employed in coating compositions, because they tend to yellow. The yellowing is, of course, immaterial in foundry resins but is unacceptable in a coating composition, particularly a decorative coating composition.
The yellowing is believed to be a result of the formation quinanoid type structures by the action of ultraviolet light in accordance with Equation III: ##STR6##
Another problem with such phenolic resins in coating applications is the poor flexibility of the coating due to the inflexible aromatic rings joined by shorter hydrocarbon chains. Also the pot life of the phenolic resin compositions is short when mixed with the isocyanates, the compositions do not produce coatings of adequate gloss for top coat application, and the coatings have poor chemical resistance particularly to caustic materials.
Various attempts have been made to overcome certain of these problems, for example, in Canadian Pat. No. 1,005,943, issued Feb. 2, 1977, David D. Taft; U.S. Pat. No. 3,789,044 issued Jan. 29, 1974 and U.S. Pat. No. 3,836,491, issued Sept. 17, 1976, both of David D. Taft et al. The Canadian Patent is concerned with diphenolic acid capping compounds; U.S. Pat. No. 3,789,044 is concerned with hydroxybenzoic acid capped epoxides; and U.S. Pat. No. 3,836,491 is concerned with capping a hydroxy-containing prepolymer with hydroxybenzoic acid and curing the capped, prepolymer with a polyisocyanate.
Although these developments have provided coating compositions of improved characteristics as compared with the compositions previously employed in the foundry art, they have nevertheless not been entirely satisfactory. In particular these compositions do not produce coatings having a satisfactory gloss, and as a result they cannot be employed as top coats. In practice they have been used only as base coats and in filler coats for porous substrates. The industry requires top coat compositions of high gloss and prior to the present invention there has been no top coat composition of adequate gloss which can be rapidly cured by a tertiary amine.
Thus, existing manufacturers have required the use of quite different coating compositions for the base coats and top coats in, for example, the manufacture of decorative articles such as those described in Canadian Pat. No. 981,124, issued Jan. 6, 1976.
There has thus been a need for a single chemical coating composition which can be rapidly cured with a tertiary amine, and that can be employed to provide a fill coat composition, a base coat composition and a top coat composition.
In addition the flexibility of the prior coating compositions of Taft has not been completely satisfactory and compositions such as those of Taft Canadian Pat. No. 1,005,943 have poor caustic resistance if there are uncured aromatic hydroxyl groups, owing to the resultant ionization.
Finally, the large size of the diphenolic acid capping compound in Canadian Pat. No. 1,005,943 produces a steric blocking hindering access of the isocyanate groups to the unreacted hydroxyl groups.